The term synthetic aperture radar (SAR) is often used to describe radar systems that use a moving antenna to simulate an extremely large antenna or aperture electronically. SAR systems are often mounted to airborne or space-based platforms and are mounted at an angle relative to the flight path of the platform to which the antenna is mounted. A monostatic SAR utilizes the same platform for the transmitter and receiver.
The flight path of the platform on which an SAR is mounted defines the azimuth direction with the antenna generally focused on a direction orthogonal to the azimuth (See FIG. 1). The direction in which the antenna is directed is often referred to as the range or slant-range. If the direction of observation is perpendicular to the direction of travel, the system is typically defined as a boresight system. Otherwise it can be called a squinted system.
In a SAR system, data is acquired by transmitting a radio pulse and receiving a signal backscattered by the imaged scene. In such systems, resolution in range increases with the bandwidth of the transmitted pulse. In many systems a frequency modulated pulse, referred to as a chirp that is a linear frequency sweep is utilized to achieve high resolution without decreasing pulse duration. Chirps are interspersed with quiescent periods for reception. In polarimetric SAR systems, chirps are typically transmitted with alternating polarities.
For space-based SAR systems, orbital speed can be so high that a large antenna is required to enable a pulse repetition rate that is sufficiently low to avoid range ambiguity. Increasing the size of the antenna reduces azimuth resolution in strip map mode. The decrease in resolution can be offset by operating in spotlight mode with a consequent loss of collection area rate. Often, the resulting physical antenna is longer than signal-to-noise considerations alone would otherwise require. The requirement of a large antenna, coupled with typically tight antenna RF precision requirements, typically means that either an expensive rocket with a large payload fairing is required, or a large, high-precision structure must be unfurled in space, raising engineering and manufacturing costs and mission risk. And indeed, space-based SAR missions launched to date typically feature either a long, often segmented antenna, or a large deployed parabolic dish.